This invention relates to a tool for replacing nuclear reactor jet pump holddown beams.
Many nuclear power generating plants, such as the two General Electric 800 megawatt units operated by the Commonwealth Edison Company at the Dresden station in Morris, Ill., utilize boiling water reactors manufactured by the Babcock and Wilcox Co.
Each of these reactors includes a vessel which is an upright cylinder approaching 70 feet in height, with both of its ends capped. The reactor vessel is filled with water. Towards the bottom of the reactor vessel lies the reactor core. The core is separated from the reactor vessel by a core shroud, which is essentially another cylindrical wall, positioned inside and coaxially with the reactor vessel. Jet water pumps are mounted in the cylindrical space between the reactor vessel and the core shroud, spaced around the shroud. The jet pumps act to direct water into the lower regions of the reactor to ensure adequate circulation. The reactor vessel cut-away view of FIG. 1 shows a jet pump assembly which includes two such jet pumps 3 mounted in the space between the reactor vessel 1 and the core shroud 2.
Each jet pump assembly includes as inlet riser 4 which is the water intake pipe of the jet pumps 3. At its upper end the inlet riser 4 connects to a head 5. The head 5 in turn is connected to two throats 7 of the jet pumps 3 via two "J"-shaped nozzles 6. Each of the two throats 7 is a vertical pipe which is connected to the narrow end of a substantially conically shaped diffuser pipe 8. Through the nozzle 6, throat 7, and diffuser 8 the water is pumped downward to the lower periphery of the reactor core (not shown).
One throat 7 lies on each side of the inlet riser 4 along the shroud 2. The inlet riser 4 is secured to the reactor vessel wall 1 by means of a brace 11. The throats 7 are secured to the inlet riser 4 by means of a gate assembly 9. The gate assembly 9 is essentially a set of two restrainer gates 10, 10 rigidly attached to the inlet riser 4 with each of the restrainer gates 10 clamped around a respective one of the throats 7.
The head 5 of the jet pump assembly has a face 13 that covers and closes the top of the inlet riser 4. The face 13 has two mouths 14 formed therein, each of which opens into the inlet riser 4. Each mouth 14 is in pressure contact with the shorter leg of one "J"-shaped nozzle 6. Each mouth 14 has two ears 15 positioned alongside it, one ear on each side of the mouth. This structure can be seen in greater detail in FIG. 2 which shows a top perspective view of the head 5. The ears 15 are vertical studs rigidly connected to the face 13, each ear 15 having a channel 16 substantially parallel to the face 13 and facing the channel in the other ear associated with that mouth, the channels 16 lying toward the top of the ears 15.
The "J"-shaped nozzle 6 of each jet pump 3 is held in contact with its respective mouth 14 by a holddown beam 17 which extends between the two ears associated with that mouth and engages the channels 16 of the ears with its end portions 18.
FIG. 3 shows a side view of one of the holddown beams 17. The body of the beam 17 is a bar 19 which tapers towards its ends and terminates in end portions 18. Protruding from the bar 19 along its length are two torquing pins 20, one on each side of the bar 19. The pins 20 are used for positioning the beam 17 and rotating it into position to engage the end portions 18 in the channels 16 of the ears 15. Mounted over the top of the bar 19 is bracket 21 through which passes a beam bolt 22. The beam bolt 22 is threaded through the bar 19 and on the underside of the bar 19 it terminates in pressure pin 23 which forms a keeper channel 24 along its periphery.
Referring back to FIG. 2, the beam 17 crosses the "J"-shaped nozzle 6 along its convex curve at a flat landing 25. The landing 25 defines a depression 25' (in FIG. 6) sized to receive the beam's pressure pin 23. Partially surrounding the depression along its edge is a "C"-shaped keeper 26 which is attached to the nozzle 6 by bolt 27. The function of the keeper 26 is to engage the keeper channel 24 of the pressure pin 23 and thus to hold the pin 23 in the depression 25' when the beam 17 is not locked into place.
When the beam 17 is positioned as shown in FIG. 2, tightening of the beam bolt 22 forces pressure pin 23 down into the depression 25' to press against the nozzle 6, thus keeping the nozzle 6 in pressure contact with the mouth 14.
Because the holddown beams 17 are held in place by the keepers 26, whenever the need has arisen in the past to remove and replace the holddown beams, it has been necessary to open the restrainer gates 10 of the gate assembly 9, to disengage the beam 17 from the ears 15, and to lift out the throat 7 and the "J"-shaped nozzle 6 along with the beam 17 as a unit. Only in this way could access be gained to the keeper 26 and the beam 17 so that the keeper 26 could be removed, the beam 17 replaced, and the keeper 26 repositioned. Then the throat and nozzle unit again had to be repositioned inside the restrainer gates 10 and on top of the inlet riser head 5 and diffusers 8, the restrainer gates 10 had to be reclosed, and the beam 17 locked into engagement of the ears 15.
This operation is both difficult and time consuming, because there is limited space to work in between the core shroud 2 and the reactor vessel wall 1 where the jet pumps 3 are located. Moreover, the restrainer gates 10 are bolted closed around the throats 7 and then the bolts are tackwelded to prevent the restrainer gates 10 from coming loose, and therefore opening of the restrainer gates 10 is difficult. In addition, surface conditions of restrainer gates 10 which have been in use for a period of time may interfere with tack welding the gate assembly 9 closed, and may necessitate replacement of the gates 10--a difficult and expensive operation. Furthermore, the jet pumps 3 are submerged and therefore all work must be done below 40 feet of water.
Due to the difficulties presented by this servicing procedure, the time required to replace the beam on a single jet pump has in the past been three days, and there are 20 of the jet pumps in the GE-designed 800 MW reactor. Of course, during servicing the reactor cannot be operated, and it is estimated that each day of reactor down-time results in a loss of one quarter million dollars. Thus any improvement in the amount of time required to change a jet pump beam results in significant savings.